It is well known to produce fluid flow by various mechanical devices such as fans and pumps. It is also well known that fluid flow can be produced by causing temperature or pressure changes in a fluid as by heating a fluid or gas to cause circulation of the fluid or circulation of the vapors created by the heating process. Heating of air and water by the sun for example produces fluid flow in the atmosphere and bodies of water. Most systems for producing fluid flow require the application of external energy or power such as electricity, heat, animal power, or other energy sources. It is also known that fluids can be moved through a porous membrane by osmosis, which requires no external energy, and by reverse osmosis which requires the application of pressure.
It is also known that gases consist of large numbers of molecules that are in continuous random motion. As used herein, the term "molecule" designates that smallest particle of any gas, which with some gases consists of combined atoms or in other gases uncombined atoms. The continuous random motion of the molecules of gas results in many collisions of the molecules. These collisions occur quite frequently for a gas at atmospheric pressure, about 3.times.10.sup.9 /sec. for each molecule of air. Because of these collisions, the direction of motion of a gas molecule is constantly changing. The diffusion of a molecule from one point to another consists of many short, straight-line segments as collisions buffet them around in random directions. Diffusion is faster for light molecules than for heavy ones. The average distance traveled by a molecule between collisions is known as the "mean free path". The higher the density of a gas, the smaller the mean free path. This means that the more molecules there are in a given volume, the shorter the average distance traveled between collisions. The term "diffusion" refers to the spread of a substance through a space or through a second substance. For example, the molecules of a perfume diffuse throughout a room.
The rate at which a gas is able to escape through a tiny hole depends on the molecular mass of the gas. The process of escape is known as "effusion".
As indicated above, it is known that lighter atoms or molecules of gas have a higher average speed than heavier molecules. Thus, it is known that atoms or molecules of a lighter gas, moving at a higher average speed than a heavier gas, will pass through a hole more rapidly than will molecules of the heavier gas. This phenomenon has been used to separate gases of different weights by passing such gases through porous barriers or membranes, see Perry's Chemical Engineering Handbook, 4th ed., sec. 21, pp. 4 to 5.
The average velocities of the molecules of two gases of a mixture of the two gases are inversely proportional to the square root of their molecular weights. When the mixture is allowed to diffuse through a porous barrier into a low pressure space, the gas which has passed through the barrier is enriched in the lightest weight constituent. Separation efficiency can be improved by using many stages in a cascade. It is necessary that the flow through the barrier be by true diffusion and not by mass flow which requires that the pore diameters be on an order of magnitude of the mean free path of the molecules, or about 7.times.10.sup.-6 cm for air at atmospheric pressure. Since the mean free path is inversely proportional to the gas pressure, larger diameter pores can be used by operating at reduced pressure.
Membranes with tapered pores having diameters on the order of the mean free path are also capable of separating gases with different molecular weights. The faster moving lower molecular weight molecules move through the pores at a faster rate than the slower moving higher molecular weight molecules. Since membranes with small tapered pores utilize the random motion of molecules to move the gas through the membranes from the small ends of the pores to the large ends, as discussed in the above parent application, the pressure on the downstream side is higher than the upstream side. Thus, recycling can be obtained by simply using bypass lines across the membranes, thereby eliminating the need for a compressor operating across each membrane in a cascade.
There are many uses for apparatus that cause fluid flow including heating and cooling systems, power generation systems and fluid transfer systems, among others.
There is a need for a system for causing fluid flow that uses little or no external energy to thereby minimize the cost of producing the fluid flow and the generation of useful work provided by such fluid flow.